<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <button>点击</button>
  <script>
    const btn = document.querySelector('button')
    // 事件源.on事件=function() ,后面覆盖前面 

    btn.onclick = function () {
      alert(6666)
    }
    btn.onclick = function () {
      alert(777)
    }
    // 事件源.addEventListenser(事件,回调函数)
    btn.addEventListener('click', function () {
      alert(777)
    })
    btn.addEventListener('click', function () {
      alert(888)
    })


  </script>
</body>

</html>